The effects of lymphocyte function following brief in vitro exposure of mouse spleens to 2,3,7,8-tetrachlorodibenzodioxin were reported last year. Further studies have shown no morphological effect by this exposure via EM. Autoradiography studies were not successful. Preliminary studies have indicated that the mitogenic suppression can occur directly in vivo by splenic perfusion (via the splenic artery) with TCDD/DMSO/culture media complexes. TCDD uptake was shown to be almost complete using 14C-TCDD. Other studies involving pre/postnatal exposure of TCDD in B6C3F1 mice have suggested that the immune suppression may be due to lymphotoxicity at higher TCDD levels and a defect in T-cell maturation at lower levels. B-cells and macrophages are not involved and none of the cell types produce suppressor activity. 2,3,7,8-tetrachlorodibenzofuran appears to act under similar mechanisms (Project # ZO1 ES 30071-01 EBCB).